Atsushi MIYASHITA

The Kitomyo Schist from Kurosegawa Belt, Shikoku, has been long considered as the oldest records of subduction metamorphism in Japan, based on an early 1970s K-Ar dating of white mica. The schist consists of mafic and pelitic layers and occurs as a tectonic block within serpentinite. Reappraisal of the schist confirmed the schist is characterized by an epidote-amphibolite peak metamorphic facies. The mafic portion is characterized by zoned amphibole + epidote + chlorite + titanite phengite +/- rutile. The presences of relict rutile surrounded by titanite and the barroisitic cores of zoned amphibole suggest a high-pressure intermediate type metamorphism at the metamorphic peak (P = -0.8-1.5 GPa and T = -500-570 degrees C). The presence of Mn-rich garnet and the lack of biotite, oligoclase and paragonite also support high-pressure intermediate type metamorphism that eliminate the possibility of a typical blueschist-facies metamorphism. New SHRIMP and LA-ICPMS zircon U-Pb geochronology on a pelitic sample show detrital grains of Mesoproterozoic and Early Paleozoic ages, suggesting a maximum deposition age for the trench-fill sediment of -440 Ma. Also the U-Pb data confirmed-360 Ma overgrown rims that might have formed during the subduction zone epidote-amphibolite facies metamorphism. Reappraisal revealed that the Kitomyo Schist is not the oldest high-pressure type schist in Japan and rather comparable to the Late Paleozoic Renge Metamorphic Rocks and their equivalents in the Kurosegawa Belt. The Devono-Carboniferous high-pressure metamorphic rocks in Japan might have been paired with their coeval batholiths along the 'Greater South China' margin that was extensively eroded during later tectonic processes. (C) 2020 Elsevier B.V. All rights reserved.

<p>The Motai-Matsugataira Belt (MMB) in the eastern margin of the northeastern Japan arc comprises Paleozoic metamorphic complexes. The high-P Yamagami metamorphic rocks in the belt are subdivided into the high-grade Yamagami I and low-grade Yamagami II units. The Yamagami I metamorphic rocks consist mainly of pelitic schist, epidote amphibolite, and garnet amphibolite. The occurrence of paragonite in rutile and the chemistry of amphiboles indicate that the Yamagami I metamorphic rocks underwent the highest-grade metamorphism in the MMB. Phengite K-Ar ages of the pelitic schists and amphibolitesindicateexhumation during 322 to 287 Ma. These petrological and geochronological results for the Yamagami metamorphic rocks indicate that part of the MMB is probably an extension of the Renge Belt of southwestern Japan.</p>

We present optical multicolour photometry of V404 Cyg during the outburst from 2015 December to 2016 January together with the simultaneous X-ray data. This outburst occurred less than sixmonths after the previous outburst in 2015 June-July. These two outbursts in 2015 were of a slow-rise and rapid-decay type and showed large-amplitude (similar to 2mag) and short-term (similar to 10 min-3 h) optical variations even at low luminosity (0.01-0.1L(Edd)). We found correlated optical and X-ray variations in two similar to 1 h time intervals and obtained a Bayesian estimate of an X-ray delay against the optical emission, which is similar to 30-50 s, during those two intervals. In addition, the relationship between the optical and X-ray luminosities was L-opt alpha L-X(0.25-0.29) at that time. These features cannot be easily explained by the conventional picture of transient black hole binaries, such as canonical disc reprocessing and synchrotron emission related to a jet. We suggest that the disc was truncated during those intervals and that the X-ray delays represent the required time for the propagation of mass accretion flow to the inner optically thin region with a speed comparable to the free-fall velocity.

<p>The Kiroko Amphibolite is located between the Cretaceous Atokura nappe and the Mikabu Greenstone complex in the Kanto Mountains of Japan, and yields early Paleozoic K-Ar ages. The amphibolite consists mainly of magnesiohornblende and albite with minor amounts of titanite, apatite, and pyrite. It contains no other high-grade metamorphic index minerals. The whole-rock composition of the amphibolite is distinct from that of the surrounding Mikabu Greenstone complex, which is geochemically similar to mid-ocean ridge basalt. In this study, the Kiroko Amphibolite yielded a U-Pb zircon age of ca. 480 Ma, which is interpreted to represent the timing of magmatism. Amphibole from the amphibolite yielded an ⁴⁰Ar/³⁹Ar plateau age of ca. 430 Ma, which is considered to be the age of metamorphism. These data suggest that the Kiroko Amphibolite forms part of the Cambro-Ordovician igneous-metamorphic complexes of Japan.</p>

高木秀雄，新井宏嘉と共著

© 2016 Macmillan Publishers Limited. All rights reserved. How black holes accrete surrounding matter is a fundamental yet unsolved question in astrophysics. It is generally believed that matter is absorbed into black holes via accretion disks, the state of which depends primarily on the mass-accretion rate. When this rate approaches the critical rate (the Eddington limit), thermal instability is supposed to occur in the inner disk, causing repetitive patterns of large-amplitude X-ray variability (oscillations) on timescales of minutes to hours. In fact, such oscillations have been observed only in sources with a high mass-accretion rate, such as GRS 1915+105 (refs 2, 3). These large-amplitude, relatively slow timescale, phenomena are thought to have physical origins distinct from those of X-ray or optical variations with small amplitudes and fast timescales (less than about 10 seconds) often observed in other black-hole binaries - for example, XTE J1118+480 (ref. 4) and GX 339â '4 (ref. 5). Here we report an extensive multi-colour optical photometric data set of V404 Cygni, an X-ray transient source containing a black hole of nine solar masses (and a companion star) at a distance of 2.4 kiloparsecs (ref. 8). Our data show that optical oscillations on timescales of 100 seconds to 2.5 hours can occur at mass-accretion rates more than ten times lower than previously thought. This suggests that the accretion rate is not the critical parameter for inducing inner-disk instabilities. Instead, we propose that a long orbital period is a key condition for these large-amplitude oscillations, because the outer part of the large disk in binaries with long orbital periods will have surface densities too low to maintain sustained mass accretion to the inner part of the disk. The lack of sustained accretion - not the actual rate - would then be the critical factor causing large-amplitude oscillations in long-period systems.

林 衛，熊沢峰夫と共著

Paragonite-clinozoisite associated within garnets (pyrope(2-7) almandine(60-64) grossular(10-28) spesartine(1-10)) was newly found in the type locality of the Sanbagawa schist in the Kanto Mountains, Japan. The composite inclusions were confirmed in prograde-zoned garnet porphyroblasts in garnet-zone metapelites that have a typical pelitic whole-rock composition (Mn/Fe = 0.019, Mg/Fe = 0.381). The garnets also contain mineral inclusions of quartz, albite, phengite, chlorite, rutile, calcite, apatite, and zircon. Phengite in the metapelite yielded a K-Ar age of 67.3 +/- 1.6 Ma as the younger part of the Sanbagawa schist in this region. This paragonite-clinozoisite association provides a mineralogical index of high pressure (P)-temperature (T) metamorphism of the type locality of the Sanbagawa schist. Moreover, the clinozoisite-paragonite composite inclusions within progradezoned garnets suggest a prograde P-T path from the stability of lawsonite and albite.

Continuing the project described in Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for SU UMa-type dwarf novae mainly observed during the 2012-2013 season. We found three objects (V444 Peg, CSS J203937, and MASTER J212624) having strongly positive period derivatives despite the long orbital period (Porb). By using the period of growing stage (stage A) superhumps, we obtained mass ratios for six objects. We characterized nine new WZ Sge-type dwarf novae. We made a pilot survey of the decline rate in the slowly fading parts of SU UMa-type and WZ Sge-type outbursts. The decline time scale was found to generally follow an expected Porb 1/4 dependence, and WZ Sge-type outbursts also generally follow this trend. There are some objects which show slower decline rates, and we consider these objects good candidates for period bouncers. We also studied unusual behavior in some objects, including BK Lyn which made a transition from an ER UMa-type state to a novalike (standstill) state in 2013, and unusually frequent occurrences of superoutbursts in NY Ser and CR Boo. We applied the least absolute shrinkage and selection operator (Lasso) power spectral analysis, which has been proven to be very effective in analyzing the Kepler data, to the ground-based photometry of BK Lyn, and detected a dramatic disappearance of the signal of negative superhumps in 2013. We suggested that the mass-transfer rates did not strongly vary between the ER UMa-type state and novalike state in BK Lyn, and this transition was less likely caused by a systematic variation of the mass-transfer rate. © 2014 The Author.

We carried out photometric observations of the SUUMa-type dwarf nova ERUMa during 2011 and 2012, which showed the existence of persistent negative superhumps even during the superoutburst. We performed a two-dimensional period analysis of its light curves by using a method called "least absolute shrinkage and selection operator" (Lasso) and the "phase dispersion minimization" (PDM) analysis, and found that the period of negative superhumps systematically changed between a superoutburst and the next superoutburst. The trend of the period change can be interpreted as a reflection of the change of the disk radius. This change is in agreement with the one predicted by the thermal tidal instability model. The normal outburst during a supercycle showed a general trend that the rising rate to its maximum becomes slower as the next superoutburst is approaching. The change can be interpreted as the consequence of the increased gas-stream flow into the inner region of the disk as a result of the tilted disk. Some of superoutbursts were found to be triggered by a precursor normal outburst when the positive superhump appeared to develop. The positive and negative superhumps coexisted during the superoutburst. Positive superhumps were prominent only for four or five days after the supermaximum, while the signal of negative superhumps became stronger after the middle phase of the superoutburst plateau. A simple combination of the positive and negative superhumps was found to be insufficient for reproduction of the complex profile variation. We were able to detect the developing phase of positive superhumps (stage A superhumps) for the first time in ER UMa-type dwarf novae. Using the period of stage A superhumps, we obtained a mass ratio of 0.100(15), which indicates that ERUMa is on the ordinary evolutional track of cataclysmic variable stars. © The Author 2014.

UPb ages of detrital zircons and white mica KAr ages are obtained from two psammitic schists from the western and eastern units of the Sanbagawa Metamorphic Belt located in the SakumaTenryu area. The detrital zircons in the sample from the western unit (T1) show an age cluster around 95?Ma, and the youngest age in the detrital zircons is 94.0 +/- 0.6?Ma. The detrital zircons in the sample from the eastern unit (T5) show a main age cluster in the Late Cretaceous with some older ages, and the youngest age in the detrital zircons is 72.8 +/- 0.9?Ma. The youngest zircon ages restrict the older limit of the depositional ages of each sample. White mica KAr ages of T1 and T5 are 69.8 +/- 1.5?Ma and 56.1 +/- 1.2?Ma, respectively, which indicate the age of exhumation and restrict the younger limit on the depositional age of each sample. The results show that the western and eastern units were different in their depositional and exhumation ages, suggesting the episodic subduction and exhumation of the Sanbagawa Belt in the SakumaTenryu area. These results also suggest simultaneous existence of subduction and exhumation paths of metamorphic rocks in the high-P/T Sanbagawa Metamorphic Belt.

This paper aims to document the principal features of the ductile shear deformation of the Sambagawa metamorphic rocks in the Ogawa-Ayukawa area, Kanto Mountains by integrating geological, petrological, microstructural and kinematic data. Metamorphic zones change northward from chlorite in the south to garnet, biotite and again garnet. Lithologic layering and metamorphic zone boundaries are generally parallel.A penetrative horizontal stretching lineation trending WNW-ESE is well developed, and is almost parallel to the trend of the Sambagawa Belt. Convincing shear sense determination using shear bands, oblique shape and c-axis fabrics of quartz grains in XZ thin sections was possible for about 14% of all samples collected. Top-to-the-west sense of shear is dominant in the study area, but minor domains with opposite sense of but shear (top-to-the-east) also occur particularly in the northern and southern areas. Quartz shape fabric and shear bands in opposite limbs of small-scale recumbent fold (F_1) indicate the same sense of shear. This suggests that the shearing postdates or was synchronous with the recumbent folding. The distribution of metamorphic zoning has been interpreted as the result of a macroscopic recumbent fold with northward vergence, however, studies of the distribution of lithology, mineral assemblages and shear-sense indicators gave no evidence in support of a large-scale recumbent fold in the area. Domains showing opposite oblique quartz shape fabrics and related interference colors with the gypsum plate inserted are observed in some quartz schists. The boundaries of the domains showing inverted sense of shear indicator are subparallel to the schistosity. Quartz c-axis fabrics have cleft girdle patterns suggesting uniaxial stretching deformation. The shortest axis of each quartz grain is parallel to Y or subsidiary to Z, suggesting that not only Z-parallel shortening but also Y-parallel shortening is dominant. Minor folds (crenulation) are commonly observed in YZ sections. These lines of evidence suggest that the bulk shear flow (fundamentally westward flow) was partitioned into the domains with opposite sense of shear and that X-parallel stretching is dominant.These features suggest the dominant deformation in the area is a ductile flow with partitioning into kinematically distinct domains and associated with overall uniaxial extension. This ductile flow is probably related to a tectonic framework such as oblique subduction of oceanic plate relative to continental plate during and after the peak metamorphism of the Sambagawa Belt. No evidence of dip-slip ductile to brittle shear which support the extrusion model of the high-P metamorphic rocks has been found in the study area.